Completed the game on Hard difficulty
Completed all levels with 3 stars
Fully leveled up the character

Path:

Saves for the game are stored in the registry, not in a separate file like most games
The default save path is:

HKEY_CURRENT_USER\Software\Awfully Nice Studios\The Bug Butcher\
Make a backup of your branch
If the branch is located elsewhere, adjust the path in the .reg file
Import the entries from the downloaded .reg file
Done. If everything is done correctly, the game will pick up your saves

Author: SuN1Sh1nE